1. Field of the Invention
The present invention relates to an uninterruptible power system (UPS), and more particularly to a power system with two current conversion units, such that when one unit fails the other one replaces the abnormal loop thus ensuring the stability of the power system.
2. Description of Related Art
With reference to FIG. 5, a conventional UPS with single current conversion module includes two power supply loops to provide an operating voltage to a load. The first power supply loop is composed of a first transformer (70), a current conversion module (71), a switching element (72) and an autotransformer (73).
A by-pass switch (74) is coupled between the output of the transformer (70) and the switching element (72). When the first power supply loop is interrupted, the by-pass switch (74) becomes conductive thus allowing the AC voltage supply to pass to the load directly.
The current conversion module (71) has a rectifier (711), an inverter (712), a battery set (75) and a battery monitoring controller (713), wherein the rectifier (711) is for converting the AC voltage to a DC voltage. In contrast with the rectifier (711), the inverter (712) converts the DC voltage to an AC voltage. The DC voltage output of the rectifier (711) is further applied to charge the battery set (75). Therefore, once the input AC voltage is unexpectedly interrupted, the battery set (75) still can provide an AC voltage to the load through the DC/AC conversion by the inverter (712).
The second power supply loop in the UPS system is formed by a second transformer (77) and another by-pass switching element (78), through which the input AC voltage can be by-passed to the load directly.
From the foregoing description, it is noted that there is only one current conversion module (71) applied in the UPS system. When the rectifier (711) or the inverter (712) has a breakdown, the UPS may experience possible loss of its function of supplying backup voltage. Although the load still derives the power supply through the second power supply loop from the input AC voltage, it may cause a great loss when the input AC voltage suddenly fails. Therefore, even the of single module UPS possesses the advantages of simple circuit design and low equipment cost, such a power system may encounter difficulties in trying satisfy the desired stability requirement.
With reference to FIG. 6, another kind of conventional UPS is provided with two current conversion modules (71)(71′) coupled in parallel. In the normal status, both of the modules (71) are operated together to supply a voltage to the load. When the rectifier (711)(711′) or the inverter (712)(712) in either module (71)(71′) breaks down, that module will be completely shutdown and the other normal module will operate individually. In a condition that both modules (71)(71′) fail, the by-pass switching element (74) turns to conductive so that the input AC voltage directly passes to the load.
There is no doubt that the stability of the UPS shown in FIG. 6 is superior to that of FIG. 5, however, the latter has high equipment cost. Another problem is that the utilization efficiency of the two modules (71)(71′) is quite low. For example, if one current conversion module (71) is shut down, it is possible that only one element of the module (71) and the rest elements are still able to operate well. However, the suspension in operating of those normal elements would result in the decrease of the utilization efficiency. Therefore, it is desired to provide a novel power system to obviate the aforementioned drawback.